A high-transparency glass having a high visible light transmittance (so-called white sheet glass) is in demand in various uses. For example, in building uses (interior materials and exterior materials), electronic equipment uses (a light guide material for a planar light-emitting device, so-called light guide plate), and other industrial uses (a cover glass for a solar power generation module), there are using methods for efficiently transmitting a visible light to enhance light utilization efficiency, for utilization as a material that provides a high design effect (high grade feeling), and the like.
In the field of use where an acrylic plate has been hitherto used as a light guide plate, in the case where a high-transparency glass is applied as a light guide plate, there have been revealed problems that light absorption inside the glass in the visible light region (wavelength: 380 to 780 nm) cannot be ignored as light path length increases and thus a decrease in luminance and in-plane luminance/color unevenness occur. In addition, it has also been revealed that product properties are remarkably lowered even by a small amount of bubble defects.
A main factor of the light absorption is an iron ion contained as an impurity. The iron ion exists as a divalent one (Fe2+) and a trivalent one (Fe3+) in a glass but particularly problematic one is Fe2+ that has broad absorption in the wavelength of 490 to 780 nm.
Fe3+ has an absorption band in the wavelength of 380 to 490 nm but the influence thereof is small since an extinction coefficient thereof per unit concentration is small by one digit as compared to that of Fe2+. Therefore, in order to reduce the light absorption in the visible region, there is necessary a means for lowering the ratio of the Fe2+ content to the total iron ion content in the glass as far as possible, that is, for decreasing redox.
In an industrially manufactured glass plate, it is substantially difficult to reduce the iron content contained as an impurity to such a degree that the transmittance of the glass plate becomes the same as that of an acrylic plate, and thus, in order to solve the above problem under the constraint condition, it is inevitable to lower the redox more than before.
Since it is known that the redox increases as the melting condition of the glass becomes higher temperature due to the influence of heat reduction, glass melting at a lower temperature is preferable for redox lowering. On the other hand, when the melting temperature of the glass is lowered, fining at melting remarkably decreases and bubble quality of the glass to be produced cannot be maintained.